Field of the Invention
The present invention relates to time-of-flight (TOF) laser range finders that exploit reflection of pulsed laser light and in particular to erroneous detection restraining circuits for laser range finders capable of preventing close-range erroneous detection caused by fog, rain, dirt on a window face of a cover, and other like factors to a minimum possible.
Related Art
The inventor of the present invention previously suggested, in JP 5092076 B (hereinafter, “Patent Document 1”), a laser area sensor that can accurately detect an intruder or the like regardless of the installation location and weather conditions by eliminating, to the highest degree possible, the negative effects on the laser light during bad outdoor weather conditions or the like, and that can prevent erroneous detection to the highest degree possible.
In this invention disclosed in Patent Document 1, the third embodiment focuses on a solution to dense fog issues as described in reference to FIGS. 6(a) to 6(f). The third embodiment includes: a second laser range finder that emits pulsed laser light and measures a period of time for light reflected by at least one object that is present in a laser light emitting direction to return thereby to obtain distance information to the object and light-reception level information of the reflected light and time width information along the time axis of the reflected light; a scanning mechanism portion that changes a measurement direction of the second laser range finder; an information acquiring portion that defines a detection area and acquires distance information, light-reception level information and time width information in each measurement direction in the detection area in a time-series manner, by the second laser range finder periodically performing measurement while the scanning mechanism portion changes the measurement direction; an information correcting portion that performs correction with a second information correcting function that, if there is a discontinuous change exceeding a predetermined amount when the distance information and light-reception level information acquired by the information acquiring portion in each measurement period are compared with distance information and light-reception level information of measurement directions adjacent to the measurement direction, removes distance information corresponding to the discontinuous change in the measurement direction in the measurement period, and with a third information correcting function that, when a measurement direction range in which an amount of change between adjacent measurement directions all falls within a predetermined range is referred to as a detection angle width when the distance information, light-reception level information and time width information acquired by the information acquiring portion in each measurement period are compared with distance information, light-reception level information and time width information of a plurality of measurement directions adjacent to the measurement direction, removes specific distance information if light-reception level information, time width information and detection angle width that correspond to the specific distance information in the measurement direction satisfy a predetermined relationship; a human body judging portion that extracts a portion that is presumed to correspond to a human body, from the distance information corrected by the information correcting portion, and judges whether or not the extracted portion matches a human body based on a time-series moving status of the extracted portion; and a human body detection signal output portion that outputs a human body detection signal if the human body judging portion judges that a human body is present.
The inventor of the present invention also previously suggested, in JP 5439684 B (hereinafter, “Patent Document 2”), a laser scanning sensor capable of maintaining reliability of detection of intruders or the like as much as possible even when the cover of a light receiving portion is dirty.
This laser scanning sensor includes: a housing in which an opening portion is formed; a cover that is arranged so as to cover the opening portion and that can transmit laser light; a laser range finder that is arranged inside the housing, includes a laser light emitting portion for emitting laser light to outside of the housing through the cover and a laser light receiving portion for receiving laser light and outputting a signal according to the amount of the received light, and performs measurement by acquiring distance information to at least one object through measurement of a period of time taken for reflected light caused by the laser light emitted by the laser light emitting portion being reflected by the object to return, and acquiring received light level information of the reflected light; a scanning mechanism portion that changes a measurement direction performed by the laser range finder; an information acquisition portion that forms a detection area, as a result of acquisition of the distance information and the received light level information by the laser range finder being periodically performed while the measurement direction is being changed by the scanning mechanism portion, and that acquires, in time series, the distance information and the received light level information for each measurement direction in the detection area; a storage portion that stores, for each measurement direction, the distance information that corresponds to a farthest side out of the distance information for the measurement direction acquired by the information acquisition portion; a determination portion that determines whether a state in which measurement directions whose distance information corresponds to a near side of a predetermined distance and whose received light level is greater than or equal to a predetermined threshold occupy a predetermined proportion or more of all measurement directions has continued for at least a predetermined period of time; and an alert output control portion that outputs an alert signal according to a result of the determination performed by the determination portion, wherein the predetermined threshold to be compared with the received light level in the determination portion is changed based on the distance information stored for each measurement direction in the storage portion.
In a dense fog condition, an extremely large number of water droplets that are much smaller than raindrops, snowdrops and the like are suspended collectively in the air, and the pulsed laser light is diffused and reflected by the large number of small water droplets. As a result, in signal waveforms of received light, pulses, such as Pulse P71, Pulse P72, Pulse P73 and Pulse P74 shown in FIGS. 6(a) to 6(f) of Patent Document 1, appear in which the light-reception level hovers around a level lower than that of another reflection and the pulse time width along the time axis having a wider shape continuously exists for a relatively long period of time. Pulses with a substantially similar shape are present in adjacent measurement directions, and a substantially similar pulse condition is present over a continuously wide angle range.
In contrast, if dirt is attached to, for example, an outer portion of the cover of the light receiving portion of the laser range finder, not the whole of laser light emitted from the light emitting element is transmitted through the cover, and part of the laser light is slightly reflected by the dirt, which may reach a light receiving element. Even if it is only a small portion of the laser light that is reflected, since the distance to the cover is extremely short, the received light level of the reflected light may have a magnitude that is not ignorable. For example, a pulse waveform having a comparatively low peaked shape, such as a waveform Wx shown in FIGS. 5(a) to 5(c) of Patent Document 2, appears on the left side, which corresponds to the near side.
These conventional techniques in most cases determine presence of fog, dirt on the cover, and the like in accordance with the light-reception level (received light level) of reflected light.
However, for example, if there exist fog and dirt on the cover simultaneously in a particular measurement direction, further presence of a human body appears as a third signal on the time axis. An extremely complex process becomes inevitable to distinguish a human body from, for example, dirt on the cover and fog, which is an obstacle to achieving desirable precision in human body detection.